1. Technological Field
The present invention relates to a wheel slip control system for a vehicle operable to control the frictional force between the driven wheel tire and the road surface during a braking operation and during an accelerating operation as well, and particularly to a wheel slip control system which controls the rotation of the driven wheel using a brake hydraulic system.
2. Related Art
There have been known so-called anti-skid brake systems for vehicles which control the rotation of the wheel so that an optimal braking force is produced without the occurrence of "locked wheel" during the braking operation. In these systems, the rotational speed of the wheel (termed "wheel speed" hereinafter) is controlled to amount to slightly lower than the running speed of the vehicle (termed "vehicle speed" hereinafter) during the braking operation so as to maximize the frictional force between the wheel tire and the road surface.
Defining a slip rate S as a function of the vehicle speed Vs and the wheel speed V as: EQU S=[(Vs-V)/Vs].times.100(%)
the frictional force M between the wheel tire and the road surface reaches a maximum, and at the same time the side force F for preventing a skid has a proper value, when the slip rate S is around 10%. Based on this fact, the wheel speed V is controlled to a speed calculated by the following equation so that the slip rate S becomes around 10%, for example: EQU V=(1-.alpha.)Vs-.beta.
where .alpha. is a constant taking a value, e.g., 0.03, and .beta. is a constant taking a value, e.g., 4 km/hour.
Various anti-slip and anti-skid wheel control systems have been developed and practically installed on vehicles for the security purpose of minimizing the stopping distance without a skid when the driver takes an abrupt braking action, whereas the development of a traction control brake system for providing an optimal vehicle accelerating performance is still inert with some attempts being made to reduce the engine output power upon detection of a wheel slip during the accelerating operation.
The conventional traction control brake system controls the output of the internal combustion engine through the control of the ignition timing or air-fuel mixing ratio, and such an operating scheme can cause an abnormal vibration and impaired exhaust emission control due to abrupt changes in the operating condition of the engine. Moreover, reduction of the engine output torque has a time lag against a change in the ignition timing or air-fuel ratio, and therefore a high response acceleration control cannot be achieved.